Clean the mushrooms (rub with kitchen paper - not water) and cut into slices. Melt the butter in the pan and fry the mushrooms in it for about 15 minutes. Then cut the onions into half rings and cook for about 5 minutes. Add the tuna and mix with the Italian herbs, salt, pepper, tomato paste and salsa sauce. Let cook for another 10 minutes over medium heat.
The dish can be eaten cold as well as hot and tastes best with toasted baguette or garlic baguette. Sounds simple, but tastes great.